The efficiency and effectiveness of many industrial processes depends on the temperatures at which the processes are performed. For example, a kiln's ability to harden or dry substances depends, at least in part, upon the kiln's temperature. Similarly, the efficiency of a furnace in converting fuel into thermal energy, and the ability of a recovery boiler to extract chemicals from waste material, is temperature-dependent.
To control a process performed by such a system, the temperature of the system must be monitored. Information is collected regarding the temperature of the area in which the process is actually being performed. This process area may include solid, liquid, and gaseous objects whose temperatures are to be measured. In some instances, a general indication of the temperature of the process area is desired. In other cases, information regarding the spatial distribution of temperature over the process area is needed.
One of the simplest ways that information regarding the temperature of a process area can be obtained is through direct visual observation of the area. For visual observation of temperature patterns to be possible, the process area must be sufficiently hot to emit radiation that can be seen by the operator. The temperature of the process area is then interpreted based upon the color or wavelength of the emitted radiation.
In that regard, the wavelengths and intensity of the emitted radiation depend upon the temperature and material composition of objects included in the area. For a given material, the wavelength corresponding to the peak spectral intensity is inversely proportional to temperature. By observing the apparent color of the process area, some indication of temperature can, thus, be obtained.
Direct visual observation also advantageously provides the operator with a good deal of nonthermal information that may be useful in controlling a process. For example, by directly viewing the process, an operator may be able to monitor objects whose location or size, rather than temperature, influence the process. The use of direct visual observation to monitor process temperature does, however, have several limitations.
More particularly, the process may take place at temperatures that are too high to allow direct visual observations to be easily or safely conducted. Further, direct visual observations often provide, at best, rough qualitative estimates of temperature that may be entirely inadequate for the required process control. In still other instances, temperature information must be collected from a number of sites, limiting the ability of a single individual to directly observe the operation at each site.
To overcome the limitations of direct visual observation, a variety of remote monitoring systems have been developed. For example, a simple system includes a camera connected to a remote monitor or display. The camera focuses radiation from the process area to produce an optical image that is converted to a video image of the area.
While a camera responsive to visible radiation may be used, in many applications the intensity of infrared emissions from the process area will be greater than emissions in the visible portion of the spectrum. Further, environmental factors related to the process environment may interfere with infrared emissions less than visible emissions. For these reasons, an infrared camera may be used to produce a video image representative of the intensity of received infrared radiation and, hence, the temperature of the viewed area.
In either case, the video image is transferred to the monitor for display. An operator viewing the monitor can then make a qualitative assessment of the way in which temperature varies over the imaged area.
Temperature monitoring systems have also been developed to provide quantitative or absolute temperature measurements. One example of such a system is the product sold by the assignee of the present application, Quadtek, Inc., under the trademark M702. The M702 system includes a pyrometer that is integrated into an infrared camera, which is, in turn, coupled to a monitor. The position of the pyrometer, relative to the camera, is fixed.
In the M702 system, the camera receives infrared radiation from the process area and provides a video output to the monitor. The pyrometer also receives radiation from a limited region of the process area viewed by the camera and provides a pyrometer output to the monitor. The monitor responds to the outputs from the camera and pyrometer by producing a displayed image of the process area and an indication of the temperature of the single region of the process area monitored by the pyrometer.
A slightly more complex system is sold by E.sup.2 Technology Corp. of Ventura, Calif., under the trademark 7000-CTS. The 7000-CTS system is a scanning temperature measurement system, which includes an infrared thermometer and a television camera. Although the position of the thermometer relative to the camera is fixed, a scanning head allows the position of the combined thermometer and camera to be adjusted. The camera is used by the operator to sight the area whose temperature is monitored by the thermometer.
Although the 7000-CTS scanning thermometer system does allow the temperature of more than one fixed region to be determined, it has several limitations. First, because both the thermometer and camera are scanned, a relatively large, bulky, and expensive scanning head is required. Further, due to the bulk of the scanned components, the scanning speed of the system is limited and may present problems when relatively continuous temperature information must be collected over large or dispersed regions of the area.
A more sophisticated temperature monitoring system is offered by Quadtek, Inc. under the trademark CALICO. The CALICO system includes a pyrometer, video imager, temperature analyzer, and monitor. The video imager is, for example, an infrared camera that produces a video image representing the intensity of infrared emissions received from the process area. The pyrometer produces a reference temperature output in response to radiation received from a single reference region included in the area viewed by the infrared camera. In operation, the CALICO imager and pyrometer pass the video image and reference temperature data to the temperature analyzer.
The temperature analyzer determines the shading or coloring of the portion of the video image associated with the reference region. The relationship between video shading and the measured reference temperature is then determined. As a result, the temperature analyzer is able to determine the absolute temperature of the imaged area at select regions of interest, other than the reference region viewed by the pyrometer, in the following manner.
The temperature analyzer first determines the location of the select region of interest on the video image. The temperature analyzer then computes the temperature associated with the shading of the video image at this point. Because the relationship between shading and temperature has already been accurately defined for the reference point, that information can then be used to calibrate a determination of temperature based upon the shade of any other point on the video image.
As will be appreciated, the output of the infrared camera of the CALICO system is used both to generate a video image of the process area and to measure temperature. The pyrometer simply provides continuous calibration of the temperature measurements based upon the camera output. Because the camera is used both to image the area of interest and to monitor temperature, its performance can typically be optimized for only one of the two tasks.
In that regard, with the camera output used for imaging, it may be desirable to alter the shading of the video image to lighten or darken specific regions of the image for better visibility. If the camera output is also used for temperature measurement, however, such shading corrections might adversely influence the interpretation of temperature. Similarly, if the displayed image generally lacks sufficient contrast, the black level or contrast of the image can be offset accordingly. Again, however, such an adjustment might adversely influence temperature measurements based upon the same image.
Another potential limitation concerning the use of the camera output for imaging and temperature measurement relates to the desired linearity or nonlinearity of the two functions. In that regard, a camera having a nonlinear response may result in good image production, but skew temperature measurements. Alternatively, a camera having a linear response may provide a truer temperature measurement, but less useful image information.
Finally, the minimum temperature that can be sensed by the CALICO system is limited by the available wavelength range that conventional infrared cameras respond to. Because most conventional infrared cameras respond only to radiation shorter than 1.8 microns, the system is typically limited to monitoring temperatures above 1470 degrees Fahrenheit (800 degrees Centigrade).
In view of these observations, it would be desirable to provide an imaging and temperature monitoring system that produces an image of the area of interest and absolute temperature information regarding select points within the area of interest, without exhibiting performance tradeoffs.